The present invention relates to an electric power steering apparatus for giving steering auxiliary power to the steering mechanism of a vehicle by a cylindrical permanent magnet synchronous motor as a brushless motor and a motor driving apparatus and more particularly to motor current control in such an electric power steering.
Electric power steering apparatus adapted for giving steering auxiliary power to steering mechanisms by driving electric motors in response to steering torque applied by drivers to handles (steering wheels) are used. As the electric motor (drive source) for use in the electric power steering apparatus, the motor with a brush has conventionally been used; however, the brushless motor such as a cylindrical permanent magnet synchronous motor is also employed in recent years in view of not only improving reliability and durability but also decreasing inertia. In the electric power steering apparatus in which the brushless motor is used, feedback control by a sine-wave electric current is performed as described hereunder.
The brushless motor normally essentially includes a rotor as a field system formed with a permanent magnet and a stator formed with a three-phase coil having U, V and W phases. In the drive control portion of the brushless motor, a voltage command value is calculated by proportional integral control operations so that an electric current having a target value that is set in response to steering torque flows through the motor and, based on the voltage command value, a sine wave voltage varying in a sine-wave form in response to the rotating position of the rotor is applied to the motor. The electric power steering apparatus is provided with a current control portion for controlling the motor current. In the current control portion, the voltage and the current as a three-phase AC current with respect to the driving of the motor are normally indicated by a rotary orthogonal coordinate system (called ‘d-q coordinates’) having a d-axis (also called an ‘orthogonal axis’) as the direction of magnetic flux of the field system of the rotor and a q-axis (also called an ‘abscissa axis’) that is perpendicular to the d-axis and leading the d-axis by π/2. With the d-q coordinates, the current made to flow through the motor can be treated as DC composed of a d-axis component and a q-axis component. In this case, the proportional integral control operations on the deviation of a target current value from the current actually flowing through the motor is carried out for each of the d-axis and q-axis components. A proportional gain and an integral gain for use in the proportional integral control operations are set on the basis of the inductance and internal resistance of the motor.
As saliency cannot be bypassed in the case of an embedded magnet synchronous motor, the d-axis quantity and the q-axis quantity of the reactance of the motor are treated separately (see Non-Patent Document 1, for example). When the embedded magnet synchronous motor is used in the electric power steering apparatus, it is necessary to carry out the proportional integral control operations based on the proportional gain and the integral gain differentiated by the d-axis component and the q-axis component of the motor current that should be kept under control.
In the case of a cylindrical permanent magnet synchronous motor, the rotor is a surface magneto structure with d-axis inductance as the d-axis quantity and q-axis inductance as the q-axis quantity of the reactance of the motor being substantially equal. Consequently, in the electric power steering apparatus using cylindrical permanent magnet synchronous motor, the proportional integral control operations is carried out on the assumption that an electrical time constant L/R as the ratio of the inductance L to the internal resistance R of the motor is set to the same value on the d-axis and the q-axis (absence of the saliency). Even in an electric power steering apparatus using a brushless motor having saliency with d-axis inductance different from q-axis inductance, moreover, the saliency can be disregarded because reluctance torque is obviated by controlling the application of voltage to the corresponding motor so that the d-axis component of the current flowing through the motor is reduced to zero.
Non-Patent Document 1
‘Designing and Control of Embedded Magnet Synchronous Motor’ by Youji Takeda et al 3, first edition, published by K. K. Ohmsha on Oct. 25, 2001
As set forth above, control of the motor current (to put it concretely, the proportional integral control operations for calculating the voltage command value) is performed on the assumption that the d-axis inductance and the q-axis inductance of the motor are equal in the electric power steering apparatus using the cylindrical permanent magnet synchronous motor.
However, it still remains unverified up to the present that the measurement of the d-axis inductance and the q-axis inductance of the motor has strictly been conducted and that the presence or absence of the difference between these factors of inductance has affected frequency characteristics of the current control in the electric power steering apparatus using the cylindrical permanent magnet synchronous motor. The present inventors looked into the foregoing problems and obtained FIG. 6 in the form of a Bode diagram showing the frequency characteristics of the q-axis current and FIG. 7 in the form of a Bode diagram showing the frequency characteristics of the d-axis current, whereby it was ascertained that there was an ignorable difference between the response of the d-axis current and that of q-axis current.